Add hooks, @file mentions, session search/fork/export, follow-scroll

- Hooks ([[hooks]]): pre_tool, post_tool, user_prompt, stop; sandboxed, exit 2 blocks
- @file mentions with autocomplete; mentioned files/folders are attached to the prompt
- Session full-text search in the sidebar, fork into a copy, export as Markdown/JSON
- Chat follows the newest message until the user scrolls up; jump-to-latest button
- SVG activity spark and thinking icon instead of Unicode/emoji glyphs
- File search ranks name matches first and returns relative paths

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/i18n/en.ts

@@ -122,6 +122,7 @@ export const en = {
   "git.commit": "Commit",
   "git.noCommits": "No commits yet.",
   "git.noDiff": "No differences.",
+  "composer.folder": "folder",
   "composer.attachedFiles": "Attached files (in the project, read them with your tools):",
   "composer.uploading": "Uploading {n}…",
   "usage.title": "Models and usage in this session",
@@ -233,6 +234,15 @@ export const en = {
   "settings.reasoningHidden": "Off",
   "settings.reasoningHint": "Thinking of the model (if the model and provider return it).",
   "settings.reasoningLive": "Open while thinking, then collapse",
+  "chat.jumpLatest": "Jump to the latest message",
+  "sidebar.search": "Search chats…",
+  "sidebar.searchResults": "Results ({n})",
+  "sidebar.noResults": "Nothing found",
+  "chat.fork": "Fork: continue in a copy of this chat",
+  "chat.forked": "Forked – you are now in the copy",
+  "chat.export": "Export",
+  "chat.exportMd": "Markdown (.md)",
+  "chat.exportJson": "JSON (.json)",
   "chat.empty": "Send a message to start.",
   "chat.compacted": "Conversation compacted – summary",
   "chat.context": "Context",
@@ -243,7 +253,7 @@ export const en = {
   "chat.delete": "Delete session",
   "chat.deleteConfirm": "Delete this session including its subagents? Statistics are kept.",
 
-  "composer.placeholder": "Message… (/ for commands, attach or paste files)",
+  "composer.placeholder": "Message… (/ for commands, @ for files, attach or paste files)",
   "composer.placeholderBusy": "Message is queued and sent at the next step…",
   "composer.send": "Send",
   "composer.queue": "Queue",